Are allergies a sign of a weak immune system? God, no. If anything, it’s the opposite. Allergies are caused by your immune system responding too strongly to something innocuous. In reality, misguided is probably a much better word. This is a good question that I think hits on a lot of subtle misconceptions so let me try to help address some of those.

Fundamentally, an allergic reaction occurs when your body mistakenly recognizes something that really isn’t bad for you as a pathogen and attacks it (like pollen). The symptoms you feel are actually due to this immune response. This is actually true in a lot of cases. Oftentimes, the symptoms you associate with an infection or wound, like a fever or swelling, are actually caused by your own immune system trying to fix you. For example, inflammation is a side effect of your own immune system sending teams of special cells to a site of distress. When doctors prescribe things like anti-inflammatories, they’re really trying to dampen your body’s immunological response.*

I should mention there’s lots of different types of allergies, or hypersensitivities, all mediated by different mechanisms. Some can be life-threatening, others are just annoying. There are four main types of hypersensitivities we traditionally call “allergies,” summarized in the chart below.

I’m going to guess the kind you’re interested in are Type I hypersensitivities (far left on the chart). These are the types of allergic reactions that are responsible for most of the common allergic reactions (the ones you usually treat with the over the counter antihistamines, i.e. hay fever). Your immune system can’t actually tell whether or not something is good or bad for you ahead of time. It decides when to operate by determining whether something is normally part of you or if it’s “foreign.”

We call what your immune systems sees antigens, which are basically little pieces of protein derived from a larger molecule, bacteria, or even your own cells. Lots of antigens represent things that are friendly, like antigens from bacteria in your gut or your own cells, and others not so much, like pathogenic bacteria. It’s up to your immune system to decide what to attack and this is really hard job. It’s sort of like trying to decide if someone is a bad guy by just looking at their fingerprint. Sometimes you might have a record of bad stuff they’ve done and decide they’re up to no good, sometimes you won’t and you’ll decide to leave them alone. This isn’t a perfect system. Sometimes you’ll mess up.

Type I hypersensitivities are mediated by a type of antibody called IgE. I think the details aren’t relevant but just know that it’s basically fast acting and binds to a special type of cell called a mast cell. Mast cells release a variety of different effector molecules useful during an immune response. Most importantly for this question, they secrete a molecule called histamine. Histamine is the dude responsible for all the things you commonly associate with allergies, like sneezing and congestion, and the reason why taking anti-histamines like Claritin alleviates your symptoms.

You might be asking now, why does my immune system recognize this stuff as foreign and why doesn’t everything I inhale cause an allergic response? The answer is long, complicated, and not fully understood but the short answer is we generally want(ed)our immune system to react this way. The immune system develops a tolerance to certain types of antigens (that’s why your immune system doesn’t constantly attack your own cells or commensal bacteria) but you obviously don’t want a tolerance to everything.

Historically, the current thinking is that people didn’t develop allergies like they do today. The same mechanism (sort of) that results in an allergic response is also very useful for fighting off extracellular parasites, like worms. Nowadays, the world we live in is so much cleaner that we don’t get the same levels of exposure to parasites so our immune system doesn’t get a chance to get “primed” and decide what’s good or bad. At least, this is the theory behind the “hygiene hypothesis.”** This is the proposed reasoning behind why Type I hypersensitivities are so much more common in the developed world than the undeveloped one. Basically, our life is so clean now that we lack exposure to certain antigens at a young age, preventing our immune system from a developing a tolerance to anything. Allergies are basically your body is mistaking something innocuous for a parasite. Note, that does not mean you should go throw your kid in the dirt so they get exposed to weird stuff nor does it give any credence to the idea you shouldn’t get vaccinated. Those would both generally be ridiculously stupid ideas.

*I’m not a doctor. This probably isn’t actually true in all cases.

**Or more specifically now, the counter-regulation hypothesis. Though I don’t know if anyone actually really calls it this since it’s more or less the same. The general idea is that getting exposure to different types of antigens early on somehow results in a tolerance to the right stuff later on.

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